For the comic and cartoon versions of Surtur, see here: Surtur (Marvel).
Surtur is an enormous fire demon and an antagonist of the upcoming film Thor: Ragnarok. However for now he only appeared as a boss in 2011 game Thor: God of Thunder, where he was voiced by Rick D. Wasserman.
Thor: God of Thunder
After Thor had unintentionally let loose the beast Mangog, Loki told him that the only way to truly defeat the creature would be to infuse his hammer with the metal known as Scabrite, which could absorb large amounts of heat. Thor travelled into the flaming realm of Muspelheim ruled by Surtur to find Scabrite metal.
Thor realized that it was Surtur's sword that had the Scabrite he needed. But Surtur had no intention give it to Thor willingly, so the son of Odin had to fight him. Surtur seeked retaliation for Odin's theft of the Eternal Flame many centuries before.
However Surtur was defeated and Thor destroyed his sword in order to collect the Scabrite he needed.
Surtur will appear in the film as an antagonist.